Ink jet printing is a non-impact method for producing images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital signals. There are various methods that may be utilized to control the deposition of ink droplets on the image-recording element to yield the desired image. In one process, known as continuous inkjet, a continuous stream of droplets is charged and deflected in an imagewise manner onto the surface of the image-recording element, while unused droplets are caught and returned to an ink sump. In another process, known as drop-on-demand ink jet, individual ink droplets arc projected as needed onto the image-recording element to form the desired image. Common methods of controlling the projection of ink droplets in drop-on-demand printing include piezoelectric transducers and thermal bubble formation. Ink jet printers have found broad applications across markets ranging from industrial labeling to short run printing to desktop document and pictorial imaging.
The inks used in the various inkjet printers can be classified as either dye-based or pigment-based. A dye is a colorant, which is dissolved in the carrier medium. A pigment is a colorant that is insoluble in the carrier medium, but is dispersed or suspended in the form of small particles, often stabilized against flocculation and settling by the use of dispersing agents. The carrier medium can be a liquid or a solid at room temperature in both cases. Commonly used carrier media include water, mixtures of water and organic co-solvents and high boiling organic solvents, such as hydrocarbons, esters, ketones, etc.
Bishop and Czekai in U.S. Pat. No. 5,679,138 describe the preparation and use of micro-milled pigments and carbons employing anionic dispersing agents. These micro-milled pigments are particularly useful in ink-jet printing because of their small particle size. Only anionic charge stabilized pigments are described. More recently, the preparation of covalently functionalized (self-dispersed) pigments and carbons suitable for ink jet printing have been described, inter alia, by Belmont in U.S. Pat. No. 5,554,739, Adams and Belmont in U.S. Pat. No. 5,707,432, Johnson and Belmont in U.S. Pat. Nos. 5,803,959 and 5,922,118 and in published applications WO 96/18695, WO 96/18696, WO 96/18689, WO 99/51690, WO 00/05313, and WO 01/51566. These publications further describe the preparation and use of ink-jet inks employing the described self-dispersed pigments. Both anionic and cationic self-dispersed pigments are described. Takada et al in U.S. Pub App 2002/0059883 described the advantages of further stabilizing cationic self-dispersed pigments with acid components. Miyabayashi, in U.S. published application 2002/0077385 describes employing distinct colored inks, each ink employing differently colored anionic polymer stabilized coloring materials or cationic polymer stabilized coloring materials in distinct ink-jet printing channels to control inter-color bleed. Katsuragi et al., in EP 1090966 and Kashiwazaki et al., in U.S. Pat. No. 6,399,674 describe employing distinct colored inks, each ink employing differently colored anionic polymer stabilized coloring materials, anionic dyes or cationic polymer stabilized coloring materials or cationic dyes in distinct ink-jet printing channels to control inter-color bleed. Earlier, Pearlstine et al. in U.S. Pat. No. 5,518,534, Looman in U.S. Pat. No. 5,679,143, Shields and Radke in U.S. Pat. No. 5,428,383 and U.S. Pat. No. 5,488,402, Wang in U.S. Pat. No. 5,772,742 and Gundlach et al., in U.S. Pat. No. 6,039,793 described approaches to control color bleed between image regions having distinct colored inks applied. Such approaches included pH adjustment or addition of multivalent metallic ions to individual colored inkjet inks. While these approaches appear to improve the inter-color bleed problem, the formation of high, uniform and consistent single color densities on a variety of plain papers as well as designed ink-jet papers have not been adequately addressed.